Many contemporary gasket applications require a gasket made from a material that is resilient yet highly resistant to corrosive chemicals and that also maintains a high tensile strength and dimensional stability at elevated temperatures and pressures. Conventional polytetrafluoroethylene (PTFE) gasketing materials have the necessary corrosion resistance, but have proven unsatisfactory in many applications because of high creep relaxation and cold flow problems that are inherent with pure PTFE. After a relatively short period of use gaskets made from conventional PTFE products are unable to withstand high bolt loads and temperatures and, as a result, lose thickness due to creep relaxation. Because the gasket no longer fills the space it once did, fluid is able to leak from the gasketed joint. The filled PTFE gasketing materials currently available outperform conventional PTFE products in most applications. However, at elevated temperatures and pressures even filled PTFE gasketing materials lack the tensile strength and dimensional stability required to avoid premature gasket failure.
An acceptable gasket for gasket applications that subject the gasketing material to prolonged periods of high temperatures and pressures without failure has not heretofore been available. An example of such an application is the batch digester commonly employed in the processing equipment used by the pulp and paper industry. This apparatus is susceptible to a problem known as uncontrolled hammering which usually occurs in a check valve and flanged joints of the batch digester. Uncontrolled hammering leads to the extrusion of filled PTFE gasketing materials from the joint and results in undesirable fluid leakage. Other applications in which gaskets are subjected to prolonged intervals of elevated temperatures and pressures are also plagued by premature gasket failure resulting from extrusion of the gasket from the joint and the subsequent loss of sealing capability.
It has been proposed to laminate PTFE to various materials, primarily metals, to enhance the effectiveness of gaskets formed from these materials. PTFE, however, does not readily adhere to metal by itself. Consequently, measures must be taken to insure that the PTFE will adhere to the metal. Hanson U.S. Pat. No. 4,670,089 is exemplary of one method of achieving adhesion between a PTFE film and a metal. This method is rather involved and requires several steps, including etching of PTFE film, separately treating a film containing a thermosetting adhesive to make it tacky, joining this film to the PTFE film, joining both films to the metal and heating the laminate thus formed while restraining the laminate from lateral expansion.
PTFE has also been bonded to metal by an adhesive material to form a composite useful as a gasketing material with improved dimensional stability. U.S. Pat. No. 4,103,913, for example, discloses a multilayer engine head gasket which resists scoring, scratching and fretting corrosion and maintains the desired shape. This laminated gasket material consists of a metal core sandwiched between intermediate layers of phosphate and outer layers formed from polytetrafluorethylene which may be mixed with molybdenum disulfide. The phosphate layer is stated to enhance the ability of the PTFE to adhere to the steel, and there is no suggestion that the gasket material described in this patent could be formed without the adhesive phosphate layer.
Other methods of bonding PTFE to metal have also been suggested. The methods described in U.S. Pat. Nos. 3,304,221 and 3,421,972 bond PTFE to metal surfaces by first roughening and cleaning the surface of the metal and applying a layer of particulate material including particulate PTFE to the metal. In the method of Pat. No. 3,304,221, a continuous film layer of polytetrafluoroethylene is then laminated over the PTFE particles. The method of Pat. No. 3,421,972 requires mixing of the PTFE particles with nickel-coated phosphorus particles prior to their application to the metal, then compressing the layer of particles to the metal at superatmospheric pressure and sintering the layer. While the laminate produced by the method of Pat. No. 3,304,221 insures that the metal is completely covered with PTFE, it relies principally upon the ability of PTFE to adhere to metal without adhesives and, therefore, is not likely to withstand the hostile environment of a paper pulping or similar operation. Further, while the addition of the nickel-coated phosphorus particles to the PTFE in the method of Pat. No. 3,421,972 enhances the tensile properties of the PTFE-metal bond, this method requires an additional stress-relieving step to obtain increased bond strength.
U.S. Pat. No. 2,976,093 discloses a method of making a reinforced PTFE laminate wherein PTFE powder is placed in a mold, a perforated metal reinforcing member is placed on the powder, and additional PTFE powder is layered on the metal member. These materials are compacted under pressure to produce a preformed composite, which is then subjected to sintering temperatures. The reinforced PTFE laminate thus produced, however, is characterized by variations in density, surface dimples and inadequate adhesion between the PTFE and the metal. Consequently, gaskets produced according to this process are susceptible to fluid leakage and internal cold flow following the application of an external load. Moreover, creep relaxation and gasket deformation at high temperatures and internal pressures may present substantial problems in gaskets produced according to the aforementioned process that will make them unsuitable for use in these types of environments.
The prior art, therefore, has failed to disclose a PTFE gasket which is characterized by low creep relaxation, minimum cold flow, excellent resistance against deformation and excellent fluid sealability under high external loads at high temperatures and pressures and in the presence of corrosive chemicals. The prior art has further failed to disclose a method of making a laminated filled PTFE-metal reinforced gasketing material having the foregoing characteristics wherein the PTFE is actually bonded to the metal without binders or adhesives.